Damping Off

One of the most depressing of problems for the gardener is when seedlings collapse and die just after they've come through.

Dr. Dorothy Derbyshire explains what exactly damping off is and how to prevent it. At the beginning of a new season, most gardeners feel a sense of expectancy.

From coloured photographs in catalogues and on seed packets they think immediately of wonderful displays of bedding plants or of tomatoes ripened on the plant and used for a salad.

However, a lot can happen between seed sowing and the final crop especially in the early stages. Seedling death, poor development or uneven growth caused by damping-off diseases are particular problems.

As more F1 hybrid seed varieties, which are more expensive than open pollinated seed become available it really is important to give the seedlings the best possible conditions for growth. Seedling collapse, or damping-off is a wide-spread problem for gardeners and commercial growers.

As seeds germinate they may be attacked or the seedlings may be infected in the post emergence stage. An affected seed tray would have a bare area of compost, probably near one end, surrounded by seedlings growing poorly. Some seedlings might have collapsed at the base with a water-soaked appearance.

After pricking off, seedlings may develop brown root tips, have a weak root system, or collapse at the stem base and fall over. This is commonly seen when tomatoes are pricked off, and the seed leaves turn a very dark green.

Causes of damping off

Damping-off diseases are caused by fungi. These microscopic organisms form colourless threads in soil, compost, or plant structures. The ones attacking seedlings are species of Pythium and Phytophthora, belonging to a group called the water moulds. This name highlights the conditions which favour the spread of these fungi. They flourish in wet compost and need water to spread from plant to plant.

In addition to the colourless threads, these fungi develop microscopic, circular or pear-shaped bodies containing minute motile spores. If a Phytophthora colony is examined under high magnification in water, it is possible to see the self-propelled spores bursting out of the pear-shaped container and swimming through the liquid by lashing two fine threads.

When the water is cold they move slowly and take a long time to come to rest, whereas at 68F (20C) the spores swim quickly but soon stop. If they rest on a root or stem surface a minute thread emerges and penetrates the plant cells. These fungi grow inside the plant and disrupt the normal cell processes.

Mycologists consider them to be a relatively primitive type of fungus because the spores are motile and require water for dispersal. Other fungi such as powdery mildews or rusts do not have this requirement.

There is another common soil-borne fungus which attacks seedlings, leading to collapse. This is called Rhizoctonia. Infected plants tend to develop a dry, reddish-brown stem rot called wire-stem, or the seedling roots may be affected. Seedlings in patches are unthrifty or pale coloured and die slowly. When conditions are moist it may even be possible to see the dark coloured fungus threads with a lens, on the plant or the compost. If a badly infected seedling is pulled up an excessive amount of soil may hang from the shriveled root indicating the weft of fungus growth on the root. Rhizoctonia does not develop spore-containing bodies like the water moulds, but spreads entirely by the threads which grow through the compost. All members of the brassica family seem particularly susceptible to infection.

Susceptible plants:

Preventing attack

Although some plants are especially prone to damage, it is best to take sensible precautions against these diseases whatever seed you plan to sow as any plant can suffer.

Before you can plan to control any plant disease, it is important to know where the fungus comes from and how it is spread. Some diseases only survive on living plants, while others can be carried inside the seed. In the case of the damping off diseases they are soil-borne but can survive in plant or soil debris. The water moulds spread most in wet compost, while Rhizoctonia is active in drier conditions.

The most fundamental principle in preventing these diseases is hygiene. The containers, such as pots and seed trays, the greenhouse and all tools must be cleaned and free of soil or plant debris. Soak the containers in hot water and scrub them thoroughly with a bristle brush to dislodge small particles of soil or debris. You may choose to use a proprietary disinfectant - if you do, make sure you ventilate the containers before use to release any remaining fumes. If you plan to stand trays on the soil floor of the greenhouse cover the soil with polythene or raise the trays up from the surface so that they don't touch the soil. You may have the advantage of a heated propagator which is excellent for germination, but watch that you put the trays afterwards on clean benching and don't expose the seedlings to a sudden change in temperature or low night temperatures.

Watering and seed compost

Besides contaminated containers the water moulds can survive in stored water. You might think that it is good to use rain water collected from your greenhouse roof and stored in a butt or tank. Certainly it is said that slightly warmed water, at the temperature of the greenhouse is best for plants. But an uncovered tank in a greenhouse soon collects plant debris, dust, and algae and spells trouble in propagation. Always use fresh mains water for moistening compost and watering seedlings. You can draw off the tap water into a can, and leave the water to warm up before use the same day.

On the seed packet there will probably be instructions on how deep to sow the seed, and the temperature for optimum germination. What the packet may not mention is the compost to use and how to manage it.

Do NOT use garden soil. would generally recommend a proprietary compost formulated for seed sowing. These composts give the seedlings the best start in life, if they are prepared correctly.

By chance, the compost may be just moist enough, as you turn it out of the bag. But it may not; and to test it, take a handful of the compost and squeeze it, open your hand and let it fall on the bench. If the compost does not bind and just hold together it is too dry. If moisture oozes between your fingers it is far too wet. To moisten the compost turn out sufficient compost onto the bench for your immediate need. Make a flat layer, and gently sprinkle water on the compost surface. Then turn and mix the compost with your hands. Do the hand test again, and continue until the compost just holds together, but drops apart as it falls on the bench. Once the compost feels right sowing can proceed. It is quite wrong to sow seeds in dry compost and then give a heavy watering. This is the way to get damping-off.

Don't use too large a container because large volumes of compost warm up slowly. When you have sown the seed as directed on the packet, only moisten the compost surface if a fine seed is involved. Cover the tray and put it at the appropriate temperature. Once the seed is surrounded by moistened soil it will germinate.

In some circumstances people use such places as the airing cupboard to germinate seeds that need high temperatures. But, alas, they may forget them and when the seeds are brought out the hapless seedlings are etiolated and unfit for the normal environment. Young plants which are grown at the appropriate temperature in light are more resistant to disease, because they are sturdy, well balanced plants. Those taken out of the airing cupboard would be much less likely to be resistant. Aim for strong, sturdy growth at moderate temperatures.

Fungicides

Commercial growers have certain fungicides which are either mixed into the compost or applied as a drench treatment before or after sowing. As yet these are not available to the amateur gardener. There are two types of treatment you can use; one is to buy a proprietary, powdered seed treatment which contains a fungicide which will reduce the possibility of infection. Shake some dust with the seed as directed and sow normally. It pays to test small batches of seed first as the dressing can harm some varieties. A liquid copper formulation or Cheshunt Compound, also with a copper base, can be used on seedlings but they can slow down the growth of very .delicate plants. If there is one type of seed which you have trouble with regularly, it could be worth finding out if young seedlings are sold ready germinated. A limited range of young plants are becoming available now from seed suppliers.

Rescuing infected plants

Finally, what happens if, despite your care, seedlings do collapse in part of a seed tray? I would not recommend salvaging the remaining seedlings which appear to look normal. They may seem unaffected, but as these diseases are soil-borne, they may not be healthy. If you transplant these seedlings some roots or rootlets must get broken, and if the fungus threads are there in the compost, then the fungus will attack the plant. If you over-water it to ''settle the plant in' then the water moulds will become active, or if you keep the compost on the dry side Rhizoctonia could infect, according to the plant you are growing.

You may have a problem with some very choice or valuable seed. In this case the salvaged seedlings should be kept in the intensive care unit! They should be potted individually into small pots of moistened compost and segregated from other plants. They should be kept warm and sprayed over with a mister regularly to keep the leaves turgid. Drench the compost of a few with copper fungicide and leave the rest alone. This treatment combined with a good growing environment should allow the roots to start growing rapidly, and you may, by this means, be able to salvage a proportion of the plants.

Dr. Dorothy Derbyshire is an expert in plant diseases with the Ministry of Agriculture.